Perpendicular magnetic recording systems typically include a recording head having a perpendicular writer and a recording medium such as a disk having a hard magnetic recording layer and a magnetic soft-underlayer (SUL). Such perpendicular magnetic recording systems may achieve recording densities of 100 or 200 Gbit/in2, or higher, but are dominated by noise due to transition jitter. This problem will need to be overcome in order to increase areal density.
One source of jitter is large grains in the media, which lead to jagged boundaries between bits. Another source of jitter is spatial and temporal fluctuations of the head field, which unintentionally shift bit transitions from their intended location on the disk. For the latter case of head field fluctuations, the demands of high areal density recording are placing stringent requirements on the write field. For example, a typical transition jitter value of 2 nm corresponds to timing jitter of 143 ps for a linear velocity of 14 m/s.
These requirements, which are challenging by themselves, are even more difficult to satisfy given that these time scales are on the order of the precessional frequencies of the magnetic materials that comprise the recording head and media. This along with the fact that the data rates are already above 1 Gbit/s call for control of the magnetization dynamics in the recording head, recording medium, and soft underlayer.
The magnetization of a ferromagnet will rotate to align itself along the direction of a magnetic field. When the magnetic field is varied at low frequencies, the magnetization will smoothly follow the field. However, when the ferromagnet is driven by a high frequency field, the magnetization no longer follows the field and, instead, can potentially undergo precession. This occurs at frequencies comparable to the resonant precessional frequency of the magnetization, which is on the order of a few GHz for the magnetic materials typically used in disk drives. Since recording data rates are approaching these natural resonant frequencies, the magnetic materials that comprise the recording system may begin to precess when writing data.
The factor that determines whether or not the magnetization will precess is the magnetic damping, which is commonly parameterized by the dimensionless constant α. This physical parameter characterizes the rate at which energy flows from the spin system to other excitations, such as phonons. In order to provide a recording system with well-behaved magnetic materials at recording frequencies, it is desirable to use materials that have relatively large α that ensures the magnetization dynamics are overdamped.
It would be desirable to provide recording systems having magnetic materials that are well behaved at recording frequencies in order to minimize jitter and improve the overall signal-to-noise ratio of the systems.